The present disclosure relates to devices and machines for printing and more particularly to the printheads and the ink supply to the printhead in such devices or machines.
As shown in FIG. 1, one type of printing machine 10 utilizes a printhead 12 that is operable to apply droplets of liquid ink onto a substrate 14 conveyed by a media transport mechanism 16. In some machines, the ink is applied directly to the substrate, as depicted in FIG. 1, while in other machines the ink is applied to a transfer element, such as a transfer drum, which then transfers the printed image onto the substrate.
The printhead 12 receives a supply of liquid ink from a remote reservoir 18. In certain machines, the ink from the remote reservoir 18 is fed under pressure to the printhead. Thus, the machine 10 may include a pressure source 20, such as a compressed air source, that is connected through a pressure valve assembly 22 to the remote reservoir 18. An output valve assembly 24 controls the flow of ink from the remote reservoir through fluid line 26 to a local reservoir 28 directly associated with the printhead 12. When ink is in the local reservoir 28, pressure from air line 30 through pressure valve assembly 22 may be applied to force the ink from the local reservoir into the printhead 12. A controller 32 controls the timing and operation of these valve assemblies, media transport mechanism and printhead of the machine 10, as is known in the art.
The printhead 12 may be configured to apply multiple colors of ink to the substrate 14. Thus, the remote and local reservoirs 18, 28 each include a plurality of separate reservoirs, one for each color of ink. The ink is typically provided in four colors—black, yellow, cyan and magenta—so that four separate reservoirs may be provided. The reservoirs may supply ink to a single printhead, or to a corresponding one of a plurality of printheads. The pressure valve assembly 22 and output valve assembly 24 will each include four valves, along with four corresponding fluid lines 26 and air lines 30. Each of the valves is individually controllable by the controller 32 to provide a multi-color printing capability at the printhead or printheads 12.
In a typical machine 10, a printhead 12 is formed by a stack of plates that define the ink flowpath through a series of manifolds between each of the local reservoirs 28 and a plurality of inkjet nozzles. The printhead stack may further include heating plates, filters and an ink discharge or diaphragm plate that is operable to eject ink through the nozzles. Pressure considerations have dictated the construction of the printhead 12 and local reservoir 28 to ensure proper printhead function. For instance, it is preferable that a slight negative pressure exist at the inkjet nozzles for the most robust or optimum ejection of ink through the nozzles. Positive pressure applied at the inkjet nozzles during printing has been found to cause nozzle failure.
In order to avoid these pressure-related problems, the local reservoir of a typical machine 10 is typically formed as a cast metal (often aluminum) tank. The reservoir in these prior devices is sized large enough (taking into account machining tolerances and tilt angles of the reservoir) so that the usable ink volume contained within the reservoir remains below the lowest row of inkjet nozzles in the printhead 12. While this approach ensures that the inkjet nozzles have a slight negative pressure during printing, it comes at a cost of higher steady-state power loss, longer warm-up times for ink contained within the large reservoir, higher material costs in manufacturing the reservoir and greater printhead weight. In addition, in this prior approach the local reservoir tank is an open system, which requires consideration of venting and ink spillage.
There is a need for a device and method for supplying ink to the printhead that meets the pressure requirements for the printhead without the costs and size associated with prior local reservoir tanks.